如何用联接替换子查询？

Question

如何获得这些子查询的内容？

（所有表的列Created和LastEdited作为时间戳）

table Process
 - ID
 - Title

table ProcessHistory
 - ID
 - ProcessID
 - HistoryID

table History
 - ID
 - Title (new, open, closed etc.) 

当我尝试获取带有最后一个状态标题的cols的进程列表时，我会这样做：

SELECT DISTINCT Process.*, History.Title AS HistoryTitle, History.ID AS HistoryID
FROM `Process`
LEFT JOIN ProcessHistory AS ProcessHistory ON Process.ID=ProcessHistory.ProcessID
LEFT JOIN History AS History ON HistoryID=ProcessHistory.HistoryID
WHERE History.ID = (
    SELECT HistoryID FROM ProcessHistory 
    WHERE ProcessID=Process.ID 
    ORDER BY ProcessHistory.ID DESC LIMIT 1
)
GROUP BY Process.ID
ORDER BY Process.ID DESC LIMIT 0, 100

当我尝试获取按特定状态过滤的列表时（最新的HistoryID为1-“所有打开的进程”）

SELECT DISTINCT Process.*, History.Title AS HistoryTitle, History.ID AS HistoryID
FROM `Process`
LEFT JOIN ProcessHistory AS ProcessHistory ON Process.ID=ProcessHistory.ProcessID
LEFT JOIN History AS History ON HistoryID=ProcessHistory.HistoryID
WHERE History.ID=(
    SELECT HistoryID FROM ProcessHistory 
    WHERE HistoryID =1   
    AND ProcessID=Process.ID ORDER BY ProcessHistory.ID DESC LIMIT 1
)
GROUP BY Process.ID
ORDER BY Process.ID DESC LIMIT 0, 100

出于性能原因，我想摆脱这些子查询？ 如何替换子查询？

提前致谢 ！

Answer 1

您发布的查询未按照ORDER BY ProcessHistory.ID DESC LIMIT 1提供正确的结果

根据查询结果尝试以下查询

 SELECT DISTINCT p.*, h.Title AS HistoryTitle,h.ID AS  HistoryID
FROM
Process p JOIN ProcessHistory ph ON p.ID=ph.ProcessID and ph.HistoryID=1 
JOIN History h ON h.ID=ph.HistoryID
GROUP BY p.ID
ORDER BY p.ID DESC LIMIT 0, 100;

这里的SQL小提琴：
sql小提琴链接

如果您想要其他结果，请发表评论。

Answer 2

SELECT *
FROM Process AS p
LEFT JOIN (
  SELECT ph.ProcessID, ph.HistoryID, h.Title AS HistoryTitle
  FROM ProcessHistory AS ph
  JOIN History AS h ON ph.HistoryID = h.ID
  WHERE h.ID = 1
) AS phh ON p.ID = phh.ProcessID
ORDER BY p.ID DESC LIMIT 100

Answer 3

由于实际的子查询用于获取与每个“进程”相关的“最后”行，因此您既不能将其转换为JOIN，也不能将其用作设置会话变量的单独查询。

Answer 4

您仍然需要进行另一个选择以过滤掉其余的过程历史记录，但是您应该使用派生表而不是子查询，例如sqlfiddle ：

SELECT Process.*, History.Title AS HistoryTitle
FROM Process
JOIN (
  SELECT ProcessID, max(HistoryID) as HistoryID
  FROM ProcessHistory
  GROUP BY ProcessID
) PH ON PH.ProcessID = Process.ID
JOIN History ON History.ID = PH.HistoryID
ORDER BY Process.ID DESC LIMIT 0, 100

正如Himanshu Patel指出的那样，您的查询“获取按特定状态过滤的列表（最新的HistoryID为1-“所有打开的进程”）将无法产生预期的效果。 它只会返回所有HistoryID为1的进程。请参见此sqlfiddle 。

相反，您希望使用派生表来获取这些最新的进程历史记录ID，将它们与进程一起加入，并过滤诸如sqlfiddle这样的历史记录ID：

SELECT DISTINCT Process.*, History.Title AS HistoryTitle
FROM Process
JOIN (
  SELECT ProcessID, max(HistoryID) as HistoryID
  FROM ProcessHistory
  GROUP BY ProcessID
) PH ON PH.ProcessID = Process.ID
JOIN History ON History.ID = PH.HistoryID
WHERE PH.HistoryID = 1
ORDER BY Process.ID DESC LIMIT 0, 100

一种替代方法是创建一个视图，该视图将ProcessHistory表筛选为每个进程的最新历史记录，然后加入该视图。 YMMV，但在某些情况下，可以通过这种方式提高性能。